1. Field of the Invention
The present invention relates to a dual spin valve sensor read head with a specular reflector film embedded in each antiparallel (AP) pinned layer next to a spacer layer and, more particularly, to such a dual spin valve sensor wherein the reflector films of the AP pinned layers reflect conduction electrons back into mean free paths of conduction electrons for increasing the magnetoresistive coefficient of the spin valve sensor.
2. Description of the Related Art
A spin valve sensor is employed by a read head for sensing magnetic fields on a moving magnetic medium, such as a rotating magnetic disk. A typical sensor includes a nonmagnetic electrically conductive first spacer layer sandwiched between a ferromagnetic pinned layer and a ferromagnetic free layer. An antiferromagnetic pinning layer interfaces the pinned layer for pinning a magnetic moment of the pinned layer 90xc2x0 to an air bearing surface (ABS) which is an exposed surface of the sensor that faces the magnetic medium. First and second leads are connected to the spin valve sensor. for conducting a sense current therethrough. A magnetic moment of the free layer is free to rotate in positive and negative directions from a quiescent or bias point position in response to positive and negative magnetic field signals from a rotating magnetic disk. The quiescent position, which is typically parallel to the ABS is the position of the magnetic moment of the free layer-with the sense current conducted through the sensor in the absence of signal fields. If the quiescent position of the magnetic moment is not parallel to the ABS in the absence of a signal field the positive and negative responses of the free layer to positive and negative signal fields will not be equal which results in read signal asymmetry which is discussed in more detail hereinbelow.
The thickness of the spacer layer is chosen so that shunting of the sense current and a magnetic coupling between the free and pinned layers are minimized. This thickness is typically less than the mean free path of electrons conducted through the sensor. With this arrangement, a portion of the conduction electrons are scattered at the interfaces of the spacer layer with respect to the pinned and free layers. When the magnetic moments of the pinned and free layers are parallel with respect to one another scattering is at a minimum and when their magnetic moments are antiparallel scattering is maximized. Changes in scattering in response to signal fields from a rotating disk changes the resistance of the spin valve sensor as a function of cos xcex8, where xcex8 is the angle between the magnetic moments of the pinned and free layers. The sensitivity of the sensor is quantified as magnetoresistive coefficient dr/R where dr is the change in resistance of the sensor between parallel and antiparallel orientations of the pinned and free layers and R is the resistance of the sensor when the moments are parallel.
The transfer curve (readback signal of the spin valve head versus applied signal from the magnetic disk) of a spin valve sensor is a substantially linear portion of the aforementioned function of cos xcex8. The greater this angle, the greater the resistance of the spin valve to the sense current and the greater the readback, signal (voltage sensed by processing circuitry). With positive and negative signal fields from a, rotating magnetic disk (assumed to be equal in magnitude), it is important that positive and negative changes of the resistance of the spin valve sensor be equal in order that the positive and negative magnitudes of the readback signals are equal. When this occurs a bias point on the transfer curve is considered to be zero and is located midway between the maximum positive and negative readback signals. When the direction of the magnetic moment of the free layer is parallel to the ABS, and the direction of the magnetic moment of the pinned layer is perpendicular to the ABS in a quiescent state (absence of signal fields) the bias point is located at zero and the positive and negative readback signals will be equal when sensing positive and negative signal fields from the magnetic disk. The readback signals are then referred to in the art as having symmetry about the zero bias, point. When the readback signals are not equal the readback signals are asymmetric which equates to reduced storage capacity.
The location of the bias point on the transfer curve is influenced by three major forces on the free layer, namely a demagnetization field (HD) from the pinned layer, a ferromagnetic coupling field (HF) between the pinned layer and the free layer, and sense current fields (HI) from all conductive layers of the spin valve except the free layer. When the sense current is conducted through the spin valve sensor, the pinning layer (if conductive), the pinned layer and the first spacer layer, which are all on one side of the free layer, impose sense current fields on the free layer that rotate the magnetic moment of the free layer in a first direction. The ferromagnetic coupling field from the pinned layer further rotates the magnetic moment of the free layer in the first direction. The demagnetization field from the pinned layer on the free layer rotates the magnetic moment of the free layer in a direction opposite to the first direction. Accordingly, the demagnetization field opposes the sense current and ferromagnetic coupling fields and can be used for counterbalancing.
In some spin valve sensors an antiparallel (AP) pinned layer structure is substituted for the typical single layer pinned layer. The AP pinned layer structure includes a nonmagnetic AP coupling layer between first and second AP pinned layers. The first AP pinned layer is exchange coupled to the antiferromagnetic pinning layer which pins the magnetic moment of the first AP pinned layer in the same direction as the magnetic spins of the pinning layer. By exchange coupling between the first and second AP pinned layers the magnetic moment of the second AP pinned layer is pinned antiparallel to the magnetic moment of the first AP pinned layer. An advantage of the AP pinned layer structure is that demagnetization fields of the first and second AP pinned layers partially counterbalance one another so that a small demagnetization field is exerted on the free layer for improved biasing of the free layer. Further, the first AP pinned layer can be thinner-than the single pinned layer which increases an exchange coupling field between the pinning layer and the first AP pinned layer. The AP pinned layer structure is described in U.S. Pat. No. 5,465,185 which is incorporated by reference herein.
Over the years a significant amount of research has been conducted to improve symmetry of the read signals, the magnetoresistive coefficient dr/R and the read gap. The read gap, which is the distance between the first and second shield layers, should be minimized to increase the linear bit reading density of the read head. These efforts have increased the storage capacity of computers from kilobytes to megabytes to gigabytes.
I have found that by embedding a thin iron oxide (FeO) film within a ferromagnetic pinning layer that conduction electrons being lost by diffusive scattering will be reflected back for spin dependent scattering thereby increasing the magnetoresistive coefficient dr/R of the spin valve sensor. In the pinned layer structure the iron oxide (FeO) film, which is 5 xc3x85 to 15 xc3x85 thick, is preferably located between first and second AP pinned films of cobalt iron (CoFe). The specular reflector film of iron oxide (FeO) is exchange coupled to each of the first and second AP pinned films. If the specular reflector film is too thick the specular reflector film pins the magnetic moments of the first and second AP pinned films by coercivity. When the AP pinned films are pinned by coercivity this makes the free layer structure less sensitive to rotation in response to signal fields from the rotating magnetic disk which equates to reduced sensitivity of the read head. Accordingly, the specular reflector film of iron oxide (FeO) must be thin.
The pinned layer may be a single multi-film pinned layer for a single pinned spin valve sensor or preferably an AP pinned layer next to a spacer layer of an AP pinned structure. While the spin valve sensor may be a single spin valve sensor with only one pinned layer with a specular reflector film it is preferred that a dual spin valve sensor is employed wherein AP pinned structures are located on opposite sides of the free layer structure with each AP pinned layer structure having an AP pinned layer with a specular reflector film. If the spin valve sensor is a single spin valve sensor it should be understood that it can be a top or a bottom spin valve sensor wherein a top spin valve sensor is where the pinning layer is located at the top of the sensor close to the second read gap layer and wherein the bottom spin valve sensor is where the pinning layer is located at the bottom of the sensor close to the first read gap layer.
The preferred iron oxide (FeO) is xcex3Fe2O3 which is a magnetic phase of iron oxide and will therefore contribute to the magnetic moment of the pinned layer. Another iron oxide is xcex1Fe2O3 which is a nonmagnetic phase of iron oxide. Accordingly, xcex1Fe2O3 will not contribute to the magnetic moment of the pinned layer. Optionally, the specular reflector film can be FeO or Fe3O4. In the AP pinned structure the first and second AP pinned films are exchange coupled to the iron oxide (FeO) specular reflector film. In contrast to typical exchange coupled films the coupling between the AP pinned and specular reflector films is substantially not unidirectional and characteristically manifests increased coercivity.
An object of the present invention is to provide a spin valve sensor which has an improved magnetoresistive coefficient dr/R.
Another object is to provide a dual spin valve sensor that returns conduction electrons to a free layer structure from both sides of the free layer structure by specular reflection.
Other objects and attendant advantages of the invention will be appreciated upon reading the following description taken together with the accompanying drawings.